1. Field of the Invention
This invention is directed generally to methods of fabricating holograms and more particularly to a method of fabricating reflective holograms for reconstruction at infrared wavelengths in silver halide emulsion films.
2. Prior Art
Infrared optical systems are well known in the prior art. These systems are used in a multiplicity of military and domestic applications. A typical system is the layer designation utilized by the Air Force for target designation. One limitation of these prior art systems stems from the necessity of employing lenses fabricated from special glass or mirrors ground to close tolerances, both of which are unduly heavy, bulky, difficult to fabricate and expensive.
Holograms have been found to be an ideal substitute in certain optical systems requiring lenses and/or mirrors. These holograms are fabricated from direct light systems and recording media and subsequently reconstructed at the recording light wavelengths.
Transmission infrared holograms have been fabricated in photographic silver halide emulsions and dichromated gelatin films by first recording images in the recording medium with visible light at a transmission hologram exposure angle .alpha.. These recordings are then used to reconstruct the image with IR light at a transmission hologram play-back angle .beta.. Previously it has not been possible to make reflective holograms with visible light for subsequent playback or reconstruction at IR wavelengths.
Reflective IR holograms have not, to my knowledge, been successfully fabricated by direct recordings in a photographic emulsion or other photosensitive film other than as taught by Applicant in U.S. Pat. No. 4,028,104, dated June 7, 1977. The reflective holograms of Applicant's prior application are limited in their use to the near IR wavelengths. The instant invention constitutes an advancement over the teachings of Applicants prior filed application by extending the image in which silver halide emulsion films may be utilized in IR applications.
In general all reflective holograms are reconstructed at identical or shorter wavelengths than that utilized to record the image in the recording photographic emulsion or film. This result may be explained in part by the teachings of E. N. Leith et al in his article entitled "Holographic Data Storage in Three-Dimensional Media", published August 1966 in Applied Optics, Vol. 5, No. 8 where the effect of shrinkage on the reflective characteristics of emulsions is discussed. Therefore other than as taught in aforesaid U.S. Pat. No. 4,028,104, the absence of direct IR recording emulsions or films has resulted in an absence of IR reflective holograms.
Reflective IR holograms designed to reflect specific IR wavelength will be useful in holographic optical elements (HOE'S) in that these holograms will reflect only specific IR radiation wavelength and the HOE would be transparent or absorptive to all other IR and visible light wavelengths.